packfile: refactor hash search with fanout table

Subsequent patches will introduce file formats that make use of a fanout
array and a sorted table containing hashes, just like packfiles.
Refactor the hash search in packfile.c into its own function, so that
those patches can make use of it as well.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/sha1-lookup.h b/sha1-lookup.h
index cf5314f..7678b23 100644
--- a/sha1-lookup.h
+++ b/sha1-lookup.h
@@ -7,4 +7,26 @@ extern int sha1_pos(const unsigned char *sha1,
 		    void *table,
 		    size_t nr,
 		    sha1_access_fn fn);
+
+/*
+ * Searches for sha1 in table, using the given fanout table to determine the
+ * interval to search, then using binary search. Returns 1 if found, 0 if not.
+ *
+ * Takes the following parameters:
+ *
+ *  - sha1: the hash to search for
+ *  - fanout_nbo: a 256-element array of NETWORK-order 32-bit integers; the
+ *    integer at position i represents the number of elements in table whose
+ *    first byte is less than or equal to i
+ *  - table: a sorted list of hashes with optional extra information in between
+ *  - stride: distance between two consecutive elements in table (should be
+ *    GIT_MAX_RAWSZ or greater)
+ *  - result: if not NULL, this function stores the element index of the
+ *    position found (if the search is successful) or the index of the least
+ *    element that is greater than sha1 (if the search is not successful)
+ *
+ * This function does not verify the validity of the fanout table.
+ */
+int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo,
+		 const unsigned char *table, size_t stride, uint32_t *result);
 #endif